The present disclosure is related to marking and printing systems, and more specifically to an element of such a system having a controlled surface roughness and oleophobicity.
Offset lithography is a common method of printing today. (For the purposes hereof, the terms “printing” and “marking” are interchangeable.) In a typical lithographic process an imaging plate, which may be a flat plate-like element, the surface of a cylinder, or belt, etc., is formed to have “image regions” formed of hydrophobic and oleophilic material, and “non-image regions” formed of a hydrophilic material. The image regions are regions corresponding to the areas on the final print (i.e., the target substrate) that are occupied by a printing or marking material such as ink, whereas the non-image regions are the regions corresponding to the areas on the final print that are not occupied by said marking material. The hydrophilic regions accept and are readily wetted by a water-based fluid, commonly referred to as a fountain or dampening fluid (typically consisting of water and a small amount of alcohol as well as other additives and/or surfactants to reduce surface tension). The hydrophobic regions repel dampening fluid and accept ink, whereas the dampening fluid formed over the hydrophilic regions forms a fluid “release layer” for rejecting ink. Therefore the hydrophilic regions of the imaging plate correspond to unprinted areas, or “non-image areas”, of the final print.
The ink may be transferred directly to a substrate, such as paper, or may be applied to an intermediate surface, such as an offset (or blanket) cylinder in an offset printing system. In the latter case, the offset cylinder is covered with a conformable coating or sleeve with a surface that can conform to the texture of the substrate, which may have surface peak-to-valley depth somewhat greater than the surface peak-to-valley depth of the blanket. Sufficient pressure is used to transfer the image from the blanket or offset cylinder to the substrate.
The above-described lithographic and offset printing techniques utilize plates which are permanently patterned with the image to be printed (or its negative), and are therefore useful only when printing a large number of copies of the same image (long print runs), such as magazines, newspapers, and the like. These methods do not permit printing a different pattern from one page to the next (referred to herein as variable printing) without removing and replacing the print cylinder and/or the imaging plate (i.e., the technique cannot accommodate true high speed variable printing wherein the image changes from impression to impression, for example, as in the case of digital printing systems).
Lithography and the so-called waterless process provide very high quality printing, in part due to the quality and color gamut of the inks used. Furthermore, these inks, which typically have very high color pigment content, are very low cost compared to toners and many other types of marking materials. Thus, while there is a desire to use the lithographic and offset inks for printing in order to take advantage of the high quality and low cost, there is also a desire to print variable data from page to page.
One problem encountered is that offset inks have too high a viscosity (often well above 50,000 cps) to be useful in typical variable printing systems such as nozzle-based inkjet systems. In addition, because of their tacky nature, offset inks have very high surface adhesion forces relative to electrostatic forces and are therefore very difficult to manipulate onto or off of a surface using electrostatics. (This is in contrast to dry or liquid toner particles used in xerographic/electrographic systems, which have low surface adhesion forces due to their particle shape and the use of tailored surface chemistry and special surface additives.)
Efforts have been made to create lithographic and offset printing systems for variable data in the past. One example is disclosed in U.S. Pat. No. 3,800,699, incorporated herein by reference, in which an intense energy source such as a laser is used to pattern-wise evaporate a dampening fluid.
In another example disclosed in U.S. Pat. No. 7,191,705, incorporated herein by reference, a hydrophilic coating is applied to an imaging belt. A laser selectively heats and evaporates or decomposes regions of the hydrophilic coating. Next, a water-based dampening fluid is applied to these hydrophilic regions rendering them oleophobic. Ink is then applied and selectively transfers onto the plate only in the areas not covered by dampening fluid, creating an inked pattern that can be transferred to a substrate. Once transferred, the belt is cleaned, a new hydrophilic coating and dampening fluid are deposited, and the patterning, inking, and printing steps are repeated, for example for printing the next batch of images.
In yet another example, a rewritable surface is utilized that can switch from hydrophilic to hydrophobic states with the application of thermal, electrical, or optical energy. Examples of these surfaces include so called switchable polymers and metal oxides such as ZnO2 and TiO2. After changing the surface state, dampening fluid selectively wets the hydrophilic areas of the programmable surface and therefore rejects the application of ink to these areas.
In general, the dampening fluid is applied as a relatively thin layer over an image plate. A certain amount of surface roughness is required in order to retain the dampening fluid thereover. In some commercially available imaging systems, specific non-printing areas are defined by a surface with an adequate surface roughness targeted to retain the thin layer of dampening fluid. Providing surface roughness is in part a function of the material forming the imaging plate. Metal imaging plates are susceptible to a variety of texturing methods, such as etching, anodizing, and so on.
A family of variable data lithography devices has been developed that use a structure to perform both the functions of a traditional plate and of a traditional blanket to retain a patterned fountain solution for inking, and to delivering that ink pattern to a substrate. See U.S. patent application Ser. No. 13/095,714, incorporated herein by reference. A blanket performing both of these functions is referred to herein as an imaging blanket. The blanket in such devices retains a dampening fluid, requiring that its surface have a selected texture. Texturing of such a structure has heretofore not been optimized.